The present invention relates to a switched-capacitor filter, and more specifically to an offset-compensated switched-capacitor filter for TDM (time-division multiplexed) multi-channel analog signals for adaptive line equalization. The present invention is particularly suitable for application to subscriber line equalization of time division multiplexed analog signals in a digital communications network such as ISDN (Integrated Services Digital Network).
Switched-capacitor technique is currently employed to implement a filter with an operational amplifier, MOS (metal oxide semiconductor) switches and capacitors to take advantage of its excellent adaptability to fabrication of integrated circuits. In the design of switched-capacitor filters an undesirable offset component arises from the offset voltage of the operational amplifier and the clock feedthrough of the switched-capacitor filter. A voltage amplifier with offset compensation as described in Proceedings of the IEEE, "Switched-Capacitor Circuit Design", Vol. 71, No. 8 Aug. 1983, page 955, FIG. 32, is suitable for such purposes.
However, if it is desired to process TDM multi-channel analog signals through a switched-capacitor filter for automatic line equalization, a sample-and-hold circuit would be required for each channel to detect the offset component from each channel to subtract it from the output of the switched-capacitor filter. As a result, a low-pass filter would be needed for each channel for filtering out the high-frequency noise of the switched-capacitor filter which has been introduced to the sample-and-hold circuit. Since the low-pass filter must have a low cut-off frequency if the frequencies of the TDM analog signals are relatively low, the resistor and capacitor components of such low-pass filter would account for a substantial proportion of the switched-capacitor circuit. As a result, a large circuit size would be required for integration. A particular disadvantage is that low-pass filters of this type cannot be implemented by integrated circuit technology.